This proposal will focus on biochemical, molecular, and cellular aspects of acute promyelocytic leukemia by the PML-RA fusion protein. With PML-RAR. We propose that the fusion enables the formation of a RAR homodimer instead of an RAR-RCR heterodimer and that this alteration affects co-factor binding, ligand responsiveness and target gene specificity. The goal of Specific Aim I is to characterize the altered transcriptional regulation by mutant RARs. This aim will examine altered co-factor recruitment and effect of co-factors such as SMRT and N-CoR on ligand binding. We will also employ chromatin IP assays to examine the effects of RAR on nucleosome remodeling. Specific Aim II will determine the functional properties of mutant receptors in vivo and determine the extent to which forced homodimers are able to alter in vitro and in vivo differentiation properties of myeloid cells. In Specific Aim III we will characterize the regulation of apoptosis by PML, PML-RAR and RAR homodimers and a variety of POOD-associated proteins such AS CBP, SP100 and SUMO. We Will also use a retroviral vector rescue technique (with Inder Verma) to isolate anti-apoptotic cDNAs. Finally, in Specific Aim IV we will use two different types of cDNA and RNA cataloguing techniques to identify changes in mRNA populations associated with retinoid induced differentiation of APL cells. Together, these studies will provide valuable new insights into the biochemical and molecular mechanisms that underlie oncogenic activation of the retinoic acid receptor and the role of retinoids in myeloid differentiation and leukemia.